In the prior art of control apparatus for two position, motor operated actuators, it is known to use electronic circuitry to provide single fault safety. For example, for use with fuel shutoff valves operated by electric motors, Military Specification MIL-V-8608A, Amendment 2, specifies a valve electrical circuit that provides protection against inadvertent actuation of the valve motor due to electrical short circuits or electrical power failures. That is, upon the occurrence of a single fault such as an open circuit in one of the valve power inputs, or a short circuit of one of the valve power inputs to VDC, electronic circuitry is specified such that the valve remains in its last energized position. However, the subsequent occurrence of a second fault of a nature of those listed hereinbefore could cause the valve to assume the position other than that of the last energized position. In a representative application of a motor operated gate valve used in aircraft to gate fuel to the gas turbine engine propulsion system, the gate valve assuming a position other than that of the last energized position is unacceptable.
Two-fault tolerance can be achieved with existing MIL-V-8608A electronic circuitry by a parallel combination of two valves and two circuits. However, this is undesirable in that it adds undue weight and complexity to the overall valve system. Therefore, it is desirable to have a two position, motor operated valve that remains in its last energized position after the occurrence of two failures.